Provide conversion of stable span to internal span

This will allow users to use rustc span messages to display user
friendly messages.

compiler/rustc_smir/src/rustc_internal/internal.rs

@@ -11,8 +11,8 @@
 use stable_mir::mir::mono::{Instance, MonoItem, StaticDef};
 use stable_mir::ty::{
     AdtDef, Binder, BoundRegionKind, BoundTyKind, BoundVariableKind, ClosureKind, Const,
-    ExistentialTraitRef, FloatTy, GenericArgKind, GenericArgs, IntTy, Region, RigidTy, TraitRef,
-    Ty, UintTy,
+    ExistentialTraitRef, FloatTy, GenericArgKind, GenericArgs, IntTy, Region, RigidTy, Span,
+    TraitRef, Ty, UintTy,
 };
 use stable_mir::{CrateItem, DefId};
 
@@ -279,6 +279,14 @@ fn internal(&self, tables: &mut Tables<'tcx>) -> Self::T {
     }
 }
 
+impl<'tcx> RustcInternal<'tcx> for Span {
+    type T = rustc_span::Span;
+
+    fn internal(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        tables[*self]
+    }
+}
+
 impl<'tcx, T> RustcInternal<'tcx> for &T
 where
     T: RustcInternal<'tcx>,